74 INVERTEBRATE PALAEONTOLOGY 



to be regarded as immutable, sharp divisions between 

 stratigraphical stages disappeared. Biological and 

 geological systems of classification came to be moulded 

 into different, and more plastic, forms, whose lack of 

 the old-time precision proved inconvenient, but natural. 

 It is, perhaps, fortunate that subdivision of geological 

 time into its main sections was carried out by believers 

 in catastrophes and special creations, since it supplies 

 a necessary basis for arbitrary classification, and would 

 be difficult to establish with the further knowledge 

 now available. Just as the genera and species diagnosed 

 by Linnaeus and his followers are still recognized, 

 though the conception of their value has altered ; so 

 the geological periods named by Smith, Murchison, 

 Sedgwick and Lyell serve as convenient excerpts from 

 eternity. It is a striking testimony to the accuracy 

 and insight of the founders of Stratigraphy that their 

 schemes of classification, based on limited experience 

 and philosophical misconception, can still be maintained, 

 with but trifling modification, after a century of dis- 

 coveries made possible by their efforts. 



The student of Historical Geology has to reckon 

 with two factors, stratigraphical and chronological re- 

 spectively. It is necessary to bear in mind that the 

 amount of work done is dependent on, but in no sense 

 commensurate with, the time spent upon it. Thus 

 one and the same interval of time may be represented 

 by the accumulation of a thousand feet of sediment 

 in one place, ten feet in another, and the removal of 

 material by denudation in a third. While stratigraphy 

 (literally interpreted) can record and correlate events 

 that have happened in course of time, it can supply no 

 reliable evidence of the duration of time itself. Physio- 

 graphical conditions (on which deposition of strata 

 depends) are too variable, in time and space, to give 



